Intermediates for the preparation of antibacterial compounds

ABSTRACT

A novel acid of formula: ##STR1## which is termed &#34;monic acid&#34; and is prepared from pseudomonic acid, is a useful intermediate for the preparation of esters thereof.

This invention relates to chemical intermediates and in particular to anallylic carboxylic acid and salts thereof which are useful for thepreparation of a class of antibacterially active esters.

Pseudomonic acid has the structure (I): ##STR2## and is disclosed ashaving antibacterial activity in British Pat. No. 1,395,907. It has nowbeen found that the allylic carboxylic acid moiety of the molecule isuseful for preparaing other esterified derivatives.

Accordingly, the present invention provides a compound of formula (II):##STR3## or a salt thereof.

The compound of formula (II) wherein the double bond is in the Econfiguration, we have designated "monic acid" and it will be referredto as such in this specification. The corresponding Z-isomer is termed"isomonic acid". It is believed that monic acid has the absolutesterochemistry as shown in formula (IIA): ##STR4## (The numbering isshown for the tetrahydropyran ring).

The salts of compound (II) may be pharmaceutically acceptable, but neednot be, as the utility of compound (II) is as a chemical intermediate.Suitable salts of the compound include metal salt, e.g. aluminium,alkali metal salts, such as sodium or potassium, alkaline earth metalsalts such as calcium or magnesium, and ammonium or substituted ammoniumsalts for example those with lower alkylamines such as triethylamine,hydroxy-lower alkylamine such as 2-hydroxyethylamine,bis-(2-hydroxyethyl)-amine, or tri-(2-hydroxyethyl)-amine,cycloalkylamines such as bicyclohexylamine, or with procaine,dibenzylamine, N,N-dibenzyl-ethylenediamine, 1-ephenamine,N-ethylpiperidine, N-benzyl-β-phenethylamine, dehydroabietylamine,N,N'-bisdehydroabietylethylenediamine, or bases of the pyridine typesuch as pyridine, collidine, or quinoline.

The compound (II) of this invention incorporates a trisubstituted doublebond and may therefore exist in both the E (the natural) and Z (or iso)geometrical forms. It is to be understood that both geometrical isomersof the compound of formula (II) are included within the scope of thisinvention, as well as mixtures of the two isomers. However, because ingeneral the E-isomer of a particular esterified derivative of compound(II) has the greater activity, it is preferable to employ that isomer.

The compounds of the present invention may be prepared from theintermediate ketone of formula (III) by any method known to convert aketone into an α,β-unsaturated acid. One such process comprises reactinga compound of formula (III) in which the hydroxyl groups may beprotected with a compound of formula (IV) or (V): ##STR5## in whichformulae (IV) and (V) the symbols R_(a), R_(b) and R_(c) are the same ordifferent and each is lower alkyl, aryl or aralkyl, and R^(x) ishydrogen, or a carboxyl-protecting group which is removable underneutral conditions; and subsequently removing any hydroxyl orcarboxyl-protecting groups.

The preferred embodiment of this process for preparing the compounds ofthe invention comprises reacting compound (III) with compound (IV).Preferably in this case R_(a) and R_(b) are methyl or ethyl. In the casewhen compound (III) is reacted with compound (V), then R_(a), R_(b) andR_(c) are preferably all phenyl.

The reaction is usually carried out in an inert solvent such asdimethylformamide, hexane, benzene, tetrahydrofuran for example, at atemperature of from about 10° to about 100° C., preferably under aninert gas such as nitrogen. Under these conditions the reaction proceedssmoothly over a period of from a few minutes to a few hours and theproduct may be isolated by any of the usual techniques e.g. solventevaporation or anti-solvent precipitation followed by filtration. Inmany cases the reaction may be carried out in a solvent in which theproduct is insoluble and in such cases the precipitated solid may becollected by filtration. Purification of the product may be by any ofthe usual chromatographic or recrystallisation techniques.

In the case where compound (III) is reacted with compound (IV) and thegroup R^(x) is hydrogen, it is convenient to treat the compound (IV)firstly with a strong base. For example sodium hydride may be used whichproduces the disodium salt; ##STR6## which is then reacted with thecompound (III).

Alternatively, the group R^(x) may be carboxyl protecting group which isremoved after the reaction. Because of the sensitivity of the moleculeto both acid and base, such a carboxyl-protecting group must beremovable under suitably mild conditions. Suitable carboxyl-protectinggroups include the 2,2,2-trichloroethyl ester, (which may be removedwith zinc in a lower alcohol, especially methanol) phenyl,pentachlorophenyl, benzyl and t-butyl ester groups. Other suitablecarboxyl-protecting groups are silyl groups. In this case the carboxylicacid is reacted with a silylating agent such as a halosilane or silazaneof the formula:

    ______________________________________                                        L.sub.3 Si U;                                                                             L.sub.2 Si U.sub.2 ;                                                                       L.sub.3 Si N L.sub.2 ;                               L.sub.3 Si NH Si L.sub.3 ;                                                                L.sub.3 Si NH CO L;                                                                        L.sub.3 Si NH CO NH Si L.sub.3 ;                     L NH CO NH Si L.sub.3 ;                                                                    ##STR7##                                                         ______________________________________                                    

wherein U is halogen and the various groups L which may be the same ordifferent, each represents hydrogen, or alkyl, alkoxy, aryl or aralkyl.A preferred silylating agent is N,O-bis(trimethylsilyl)acetamide, whichproduces the trimethylsilyl derivative of the acid.

Prior to the above process of this invention, it may be desirable toprotect the hydroxyl groups in compound (III). Although the reactionwith the compound (IV) or (V) is possible without hydroxyl protection,in general higher yields of the product (II) are formed if the hydroxylgroups are protected. Again such protecting groups must be removableunder suitably mild conditions and suitable groups include silyl groupsproduced from a silylating agent as discussed above. Particularlysuitable hydroxyl-protecting groups include trimethylsilyl,t-butyldimethylsilyl, methylthiomethyl, A preferred hydroxyl-protectinggroup is trimethylsilyl, as it is readily removed on completion of thereaction.

The compounds (II) may also be prepared by reacting the ketone offormula (III) with:

(a) an ethynyl ether of formula (VI);

    hc .tbd. c -- or.sup.x                                     (VI)

wherein R^(x) is as defined above and subsequently treating the productwith acid;

(b) an α-lithium carboxylic acid derivative of formula (VII): ##STR8##wherein R^(x) is as defined above and R^(y) is a silyl group, preferablytrimethylsilyl;

(c) a malonic acid derivative of formula (VIII): ##STR9## wherein R^(x)is as defined above in the presence of titanium chloride and pyridine;

(d) a reagent to convert compound (V) to an enamine and subsequentlyreacting the enamine with a malonic acid derivative of formula (IX):##STR10## wherein R^(x) is as defined above.

The compound of formula (III) is a valuable intermediate and also formspart of this invention.

The compound may be produced by a process which comprises treatingpseudomonic acid of formula (I) above, or an ester thereof, with ozone.

This reaction may be performed without protecting the hydroxyl groups inpseudomonic acid and is preferably carried out at a low temperature suchas -50° to -80°, suitably -70° to -80° C.

It will be noted that the triacetate derivative of compound (III) wasdisclosed in British Patent Number 1,395,907 during the structureelucidation of pseudomonic acid. However, the compound (III) is notdisclosed therein and there is no suggestion of a method of removing theacetate groups in order to prepare compound (III).

The compound of formula (II) may also be prepared by chemical or enzymichydrolysis of an ester of a compound of formula (II) under conditionswhich do not disrupt the rest of the molecule.

Esters of the compound of formula (II) are disclosed in ourcorresponding application Ser. No. 803,466, filed June 6, 1977 and anysuch ester may be employed for hydrolysis to compound (II). Normally itis preferable to employ the naturally-occurring ester, that ispseudomonic acid of compound (I).

A particularly advantageous method of carrying out this hydrolysisprocess for the preparation of compound (II) comprises:

(a) protecting a compound of formula (X): ##STR11## wherein R representsan ester-forming radical, with a hydroxyl-protecting group which isstable to alkaline conditions and is removable under mild acidconditions;

(b) hydrolysing the ester radical --CO₂ R from resulting compound underalkaline conditions; and

(c) removing the hydroxyl-protecting group.

The choice of hydroxyl-protecting group is important in the presentprocess because the molecule of formula (II) [and (X)] is susceptable torearrangement under the alkaline conditions necessary to carry out theester hydrolysis step. It may only be necessary to protect the hydroxylgroup at position 4 on the molecule, but this is most convenientlyeffected either by protecting the glycol moiety, that is the hydroxylgroups at positions 3 and 4, by a single protecting group, or byprotecting all three hydroxyl groups in the molecule.

The choice of a suitable hydroxyl-protecting group is also important andmust (a) react readily with the hydroxy group; (b) be stable underalkaline conditions and (c) be either removable under mild acidicconditions which again do not cause rearrangement of the molecule, orconverted under mild acid conditions to a different group which isremovable under alkaline or enzymic conditions.

Preferably the glycol moiety is protected and suitable reagents forforming the hydroxy-protecting group include compounds of formula (XI):##STR12## wherein R³ is hydrogen or a C₁₋₆ alkyl group and R⁴, R⁵ and R⁶independently represent a C₁₋₆ alkyl group.

The use of the compound of formula (XI) in the hydrolysis process isillustrated in Scheme A, where X represents the residue: ##STR13##(wherein the hydroxyl group may also be protected during the reaction).##STR14##

The group R³ may be for example hydrogen, methyl ethyl, n- oriso-propyl. Most suitably, R³ represents hydrogen so that the compoundof formula (XI) is a trialkyl orthoformate. In such a case, the groupsremaining attached to the hydroxyl groups in formulae (XIVA) and (XIVB)are formyl groups and are readily removed under mild alkaline conditionsto re-generate the free hydroxyl group, without disrupting the rest ofthe molecule. If the group R³ is a C₁₋₆ alkyl group the correspondingC₁₋₆ alkanoyl protecting groups in compounds (XIVA) and (XIVB) may alsobe removed by either a chemical or enzymatic hydrolysis procedure.

The groups R⁴, R⁵, and R⁶ may be for example, methyl, ethyl, n- oriso-propyl, n- or iso-, sec- or tert--butyl. Preferably R⁴, R⁵, and R⁶are all the same and each represents a methyl group. The group R isconveniently -(CH₂)₈ CO₂ H, that is the starting material of formula (X)is pseudomonic acid.

It is pointed out that the formation of compound (XII) in Scheme Aintroduces an additional optically active centre into the molecule andthe compound (XII) is normally produced as a mixture of two epimers. Itis unnecessary to separate these epimers and the optically active centreis removed when this glycol protecting group is eventually removed.

The alkaline hydrolysis of step (b) above may be carried out by anyconventional method. Suitable bases for this step include inorganicbases, especially alkali metal hydroxides, such as sodium hydroxide,potassium hydroxide, carbonates such as potassium carbonate andbicarbonates such as sodium bicarbonate or potassium bicarbonate. Thereaction is generally carried out at ambient temperature for a period offrom 1 to 10 hours. A suitable temperature is from 20° to 80° C.preferably from 50° to 80°, especially from 60° to 70° C.

The hydroxyl-protecting group is then removed by a conventional methodfor the particular hydroxyl-protecting group, and the compound offormula (II) is isolated.

The hydroxyl-protecting group may be such that it can be removeddirectly or alternatively, it may be converted by mild acid treatmentinto a different protecting group which is then removable under alkalineconditions. This latter approach is illustrated in Scheme A wherein theglycol protecting group is converted by acid to the group -OCOR³ whichis then removed.

The hydroxyl-protecting group is then removed by a conventional methodfor the particular hydroxyl-protecting group, and the compound offormula (II) is isolated.

The compound (II) is a valuable intermediate for the preparation ofantibacterially active esters thereof as described in our co-pendingApplication Serial No. 803,466, filed June 6, 1977.

The following Examples illustrate the present invention:

EXAMPLE 1 Preparation of 2S-Acetonyl-3R,4R-dihydroxy-5S-(2S,3S-epoxy-5S-hydroxy-4S-methylhexyl)-2,3,5,6-tetrahydropyran (compound A)##STR15## Ozonised oxygen (ca 1%) was bubbled through a solution ofmethyl pseudomonate (0.514 g) in methanol (8 ml) and pyridine (2 drops)at -78° C. for 0.5 hour (when blue colour developed). The excess ozonewas blown off by dry nitrogen at -78° C. Triethyl phosphite (80%. 0.3ml) was then added and the reaction mixture was allowed to come to roomtemperature. The solvent was removed at room temperature in vacuo andthe residue was chromatographed over silica gel (20 g). Elution of thecolumn with chloroform-methanol (93:7) at the rate of 2 ml min⁻¹ gavethe title compound (0.299 g), m.p. 85°-86° (from chloroform), [α]_(D)²⁰ + 11.9° (c, 1.0, CHCl₃), νmax. (CHCl₃) 1708, 1112, 1080, and 1050cm⁻¹. EXAMPLE 2 Preparation of4-[3R,4R-dihydroxy-5S-(2S,3S-epoxy-5S-hydroxy-4S-methylhexyl)-2,3,5,6-tetrahydropyran-2S-yl]-3-methylbut-2E-enoicacid (monic acid) (a) From Pseudomonic Acid (without protection)

Sodium pseudomonate (10 mg) and potassium carbonate (15 mg) wasdissolved in water (2 ml). The resulting solution was heated to 60° C.and the reaction monitored by analytical high pressure liquidchromatography which after 11/2 hours showed that optimum conversion tomonic acid had occurred. To confirm the presence of monic acid, thereaction mixture was cooled, diluted with water (3 ml) saturated withsodium chloride, layered with ethyl acetate (10 ml) and the pH adjustedto 2.0 with rapid stirring. The organic layer was separated and theaqueous phase re-extracted with ethyl acetate (2 × 10 ml). Thecolourless ethyl acetate extracts were combined, treated with excessethereal diazomethane and evaporated to dryness. The resulting mixtureof esters were examined by h.p.l.c. in several solvent systems. Themajor peaks in the chromatogram were shown to have identical retentiontimes with authentic samples of methyl monate and methyl pseudomonate,thereby confirming the presence of monic acid together with startingpseudomonic acid in the hydrolysate.

(b) From Methyl Monate

A solution of methyl monate (10 mg) in methanol (0.5 ml) was added to asolution of potassium carbonate (15 mg) in water (0.5 ml). The combinedsolution was heated to 60° C. After 1/2 hour, comparison of peakretention times with authentic monic acid by h.p.l.c. analysis confirmedthe presence of monic acid in hydrolysate.

EXAMPLE 3 Preparation of monic acid from ketone (A) by Wittigcondensation (i) Diethyl carboxymethylenephosphonate

Triethyl phosphonoacetate (44.8 g, 0.2 M) was dissolved in 1N sodiumhydroxide solution (200 ml; 0.2 M) and stirred at room temperatureovernight. The pH was adjusted from 9.0 to 1.0 with dilute hydrochloricacid. The solution was saturated with sodium chloride and extracted withethyl acetate (3 x 100 ml). The latter was washed with saturated brine,dried over magnesium sulphate, filtered and evaporated to dryness invacuo to give a viscous, colourless oil, which crystallized to a whitesolid when kept below room temperature (37.4 g; 96%). Thin layerchromatography revealed one component in chloroform at Rf = 0.02 asvisualised with iodine vapour. n_(D) ²³ = 1.3900.δ (CDCl₃) 9.33 (1H, s,CO₂ H), 4.07 (4H, octet, Me-CH₂ -O-P, J_(HH) = 6 Hz, J_(HP) = 8 Hz),2.88 (2H, d, P--CH₂ --CO₂ H, J_(HP) = 22 Hz) and 9.25 (6H, t, CH₃ --CH₂,J = 6 Hz). Irradiation at δ 9.25 produces a doublet at 4.07 with J_(HP)= 8 Hz, νmax(film) 1730 (C=O Str.), 1230 (P=O str.), 1170 (P-O vib.),1050 (P-O vib.) cm⁻¹. (Found: C, 37.10; H, 7.07; P, 15.66%; C₆ H₁₃ PO₅requires C, 36.74; H, 6.69; P, 15.79%).

(ii) Monic acid

N,O-Bistrimethylsilylacetamide (1.52 ml; 6mM) was added to a solution of2-acetonyl-3,4-dihydroxy-5-(5-hydroxy-2,3-epoxy-4-methylhexyl)-2,3,5,6-tetrahydropyran(302 mg; 1mM) in dry acetonitrile (6 ml). The solution was stirred atroom temperature for 1 hour followed by evaporation to dryness in vacuoat 40° C. The oily residue was dissolved in dry dimethylformamide (6 ml)for use in the next stage. Sodium hydride(114 mg; 80% pure; 3.8 mM) wasadded portionwise over 1/2 hour to a solution of diethylcarboxymethylene phosphonate (392 mg; 2mM) in dry dimethylformamide (5ml) at 0° under dry nitrogen. The mixture was stirred at for a further 2hours. The solution of the silylated ketone above was added dropwise tothis mixture at 0° C. under nitrogen and the resulting reaction mixturestirred overnight at room temperature. The latter was evaporated todryness and the dark residue dissolved in water (10 ml) and ethanol (10ml) and the pH adjusted to 1.8. After 5 min., at room temperature thesolution was diluted with water (15 ml) saturated with sodium chlorideand extracted with ethyl acetate (4 × 10 ml). The latter was washed withbrine, dried over magnesium sulphate, filtered and evaporated to drynessin vacuo to give monic acid. A sample of the resulting oil mixture wasdissolved in ethyl acetate and treated with diazomethane, thusconverting the monic acid present into methyl monate. The presence ofthe latter was confirmed by 4 analytical h.p.l.c. comparisons withauthentic pure methyl monate.

EXAMPLE 44-[3R,4R-Dihydroxy-5S-(2S,3S-epoxy-5S-hydroxy-4S-methylhexyl)-2,3,5,6-tetrahydropyran-2S-yl]-3-methylbut-2E-enoicacid (Monic Acid) (with protection)

Pseudomonic acid (10g; 20mM) was dissolved in trimethyl orthoformate(50ml). p-Toluenesulphonic acid (20mg) was added and the solution wasstirred at room temperature for 1/2 hour, followed by evaporation todryness in vacuo. The resulting oil was dissolved in 1N sodium hydroxidesolution (100ml; 100mM) and the solution stirred at 65° C. for 2 hours.After completion of the hydrolysis (hplc) the solution was cooled andthe pH adjusted to 7.0 with hydrochloric acid. Methanol (75ml) wasadded, the pH was adjusted to 2.0 with 5N hydrochloric acid and thereaction mixture stirred at room temperature for 0.25 hour. The pH wasre-adjusted to 9-9.5 with sodium hydroxide solution and maintained untilcomplete hydrolysis of the O-formate (c.a. 3 hours at room temperature;hplc). The pH was adjusted to 7.0 and the solution evaporated to smallbulk (10 - 20ml), saturated with sodium chloride, layered with ethylacetate and with stirring the pH was adjusted to 3.0. The ethyl acetatelayer was separated, washed with saturated brine, dried over magnesiumsulphate and evaporated to an oil, which was dissolved in water byaddition of 1N sodium hydroxide solution to pH 7.5. The resultingsolution of sodium monate and sodium 9-hydroxynonanoate was evaporatedto dryness in vacuo (12.64g). This solid was extracted with ethanol (2 ×50ml) and filtered. The ethanol filtrate was evaporated to dryness togive sodium monate (9.62g) as a white solid. The latter was dissolved inwater with ethyl acetate and acidified to pH 3.0. The ethyl acetateextract was washed with saturated brine, dried over magnesium sulphateand evaporated in vacuo to an oil (8.48g). Trituration with dry etherafforded monic acid as a white solid, which was collected and dried(2.62g; 38%), m.p. 133°-135° C. (crystals from ethanol m.p. 146°-147°C.) (Found: C, 59.0; 8.2% C₁₇ H₂₈ O₇ requires C, 59.3; H, 8.2%). Tlcrevealed a single component Rf = 0.44 in chloroform, acetone, aceticacid 12:5:3 and a single peak by hplc [α]_(D) - 13° (c, 1.0EtOH) and-20° (c, 1.0 1% NaHCO₃), υmax (KBr) 3300, 2960, 2950, 1690, 1640, 1450,1250cm⁻¹, λmax 221nm (εm 11,200), 8_(H) (d6-DMSO) 5.55 (1H,s,═CH), 2.05##STR16## 1.05 (3H,d,>CHCH₃) and 0.80 (3H,d,>CHCH₃) 8_(C) (d⁶ -DMSO) (2signals under the DMSO peaks) 167.3, 156.4, 117.6, 74.5, 69.4, 68.2,67.7, 64.6, 59.0, 54.6, 37.3, 31,47, 20.0, 18.4 and 11.6, m/e 227 (82% ,M⁺ -- H₂ O -- C₅ H₇ O₂), 141 (43%) 111 (100%).

EXAMPLE 5 Sodium Monate

Monic Acid prepared in Example 4 (3.44g; 1mM) was dissolved in water(10ml). N/10 sodium hydroxide solution (10ml; 1mM) was added to thestirred solution until complete solution was obtained (pH 7.5). Thelatter was freeze dried and finally dried in vacuo over P₂ O₅. (3.66g;100%). [α]_(D) - 20° (c,1.0 H₂ O) υmax (KBr) 3400, 2970, 1650,1550cm⁻¹., λmax (EtOH) 214nm (εm 14,600), 8_(H) (d⁶ -DMSO) 5.16 (1H,s,═CH), 1.95 (3H,s, ═CCH₃), 1.05 (3H,d,>CHCH₃) and 0.79 (3H,d,>CHCH₃).

Improved Isolation of Monic Acid

Pure Crystalline pseudomonic acid (1.00gm; 2mM) was dissolved intrimethylorthoformate (10ml) and stirred at R.T. for 30 minutes withp-toluene sulphonic acid (10mg). The solvent was then removed at reducedpressure and the residual oil immediately dissolved in 1N NaOH (10ml;10mM). The solution was stirred at 65° C. for 3 hours, then cooled andthe pH adjusted to 7.0 with conc. HCl. Methanol (10ml) was added, the pHwas adjusted to 2.0 with 5N HCl and the solution was stirred at R.T. for15 minutes. The pH was then raised to an maintained at 9.0 - 9.5 withNaOH for 3 hours, when HPLC indicated complete hydrolysis of theO-formate. The pH was adjusted to 7.0 and the solution evaporated todryness at reduced pressure. The residual solid was dissolved in water(20ml), saturated with NaCl, layered with ethyl acetate and acidified topH 3. The organic layer was separated and the aqueous layer furtherextracted with 5 × 50ml ethyl acetate. The combined organic extractswere dried over anhydrous MgSO₄ and the solvent removed at reducedpressure to yield a yellow oil (1.377gm; 1433/50/1.) Trituration withdry diethyl ether gave the monic acid (>90% pure by HPLC and TLC) as awhite solid (0.393gm; 1433/50/2). A further 0.146gm (1433/50/3) whitesolid was obtained from the mother liquors. Total yield = 0.539gm (78%).M. pt. 130° - 133° C. The product was identical to authentic monic acidby HPLC and TLC (chloroform/acetate/acetic acid 50:50:7).

What we claim is:
 1. A compound of formula (II), or a salt thereof:##STR17##
 2. A compound as claimed in claim 1 in the form of its alkalimetal salt.
 3. A compound as claimed in claim 1 in which the double bondis in the E-configuration.